A placebo was a substance containing no medication which was administered to reinforce a patient's expectation to get well or, alternatively, used in medical experiments as a control measure.
In 2153, Dr. Phlox gave Commander Tucker a placebo after he wouldn't try Vulcan neuro-pressure therapy. (ENT: "The Xindi")
Every flu season on the USS Voyager, Neelix became convinced that he had Toluncan ague. Rather than arguing with him, The Doctor would simply give him a placebo. (VOY: "Virtuoso")